Hayek, Popper, and the Causal Theory of the Mind

In late 1952, F. A. Hayek sent his friend Karl Popper a copy of his recently published book The Sensory Order: An Inquiry into the Foundations of Theoretical Psychology. In a letter dated December 2, 1952, Popper acknowledged receipt of the book and responded as follows to what he had read in it:

I am not sure whether one could describe your theory as a causal theory of the sensory order. I think, indeed, that one can. But then, it would be also the sketch of a causal theory of the mind. But I think I can show that a causal theory of the mind cannot be true (although I cannot show this of the sensory order; more precisely, I think I can show the impossibility of a causal theory of the human language (although I cannot show the impossibility of a causal theory of perception). I am writing a paper on the impossibility of a causal theory of the human language, and its bearing upon the body-mind problem, which must be finished in ten days. I shall send you a copy as soon as it is & typed.

In a later letter dated January 19, 1953, Popper added, As to my comments on your book, they are, as far as criticism is concerned, implicit in my paper. I think you have made a splendid effort towards a theory of the sub-linguistic (¼ sub-human ((¼descriptive)) language) level of mind; but I believe that no physiological approach (although most important) can be sufficient to explain the descriptive and argumentative functions of language. Or in other words, there can be no causal or physiological theory of reason. The paper Popper was referring to is his short article ‘‘Language and the body-mind problem.’’ Hayek began a draft of a paper entitled ‘‘Within systems and about systems: A statement of some problems of a theory of communication,’’ which, as Jack Birner has suggested, appears to have been intended at least in part as a response to Popper’s criticisms. But it was never completed, and Hayek never addressed Popper’s arguments in any of his published work. The Sensory Order has, however unjustly, largely been forgotten outside the circles of Hayek specialists. Popper’s brief paper is perhaps even less well known. Neither Popper’s letters to Hayek nor Hayek’s unfinished draft have yet been published. So, this episode might seem rather insignificant in the history of thought and indeed of little significance even to our understanding of either Hayek’s thought or Popper’s. But, as I hope to show in what follows, nothing could be further from the truth. With respect both to its general themes and to some of the specific philosophical moves made by each side, the brief, private dispute between Hayek and Popper foreshadowed a more prominent debate within twentieth-century analytic philosophy that began in the 1970s and continues to this day. Moreover, both the dispute between Hayek and Popper and the later debate reflect a deep tension that has lain at the heart of Western thought since the time of the scientific revolution. On the one hand, there is the ‘‘mechanical world picture’’5 according to which all natural phenomena can be explained entirely in terms of the mathematically describable behavior of matter in motion. On the other hand, there are rational human thought processes, including the philosophical and scientific theorizing that led to the mechanical world picture itself. It is far from obvious that the latter can be fitted comfortably into the former – that human rationality can be explained in terms of purely material processes – and from the time of Descartes until relatively recently, the dominant view was that it could not be. Hayek and Popper were writing at a time when this view began to give way to a new materialist orthodoxy. Hayek, though arguably more sensitive to the tension in question than most contemporary materialists, nevertheless thought it could be resolved in a way favorable to a broadly materialist or ‘‘naturalistic’’ understanding of the mind. Popper disagreed and believed the older, dualistic conception of the mind to be essentially correct, and as we will see, his reasons for doing so have in more recent years been regarded even by some non-dualist philosophers as posing a serious difficulty for materialism. In the next section, I will set the stage for the discussion of Hayek and Popper with a brief account of the nature and origins of the mind-body problem (or ‘‘body-mind problem,’’ as Popper preferred to call it). We will see that there are really at least three mind-body problems, and that while Hayek and most contemporary philosophers focus on the first of these, Popper was more concerned with the other two and believed that they pose a more serious difficulty for materialism than the former does. The third section will explain what a ‘‘causal theory of the mind’’ is and the respects in which Hayek’s account can be regarded as a causal theory. The fourth section will examine Popper’s main criticism of causal theories, which will be elucidated by comparison with the views of contemporary philosopher Hilary Putnam, who (apparently independently) developed a line of argument that parallels and extends the one presented by Popper. Finally, in the fifth section, I will consider the possible response to Popper suggested both by Hayek’s unpublished draft and by things Hayek had to say in some of his published work, relating it to the responses contemporary philosophers have given to arguments like those presented by Popper and Putnam. I will argue that none of these replies succeeds and that the Popperian critique remains a powerful and as yet unanswered challenge not only to dogmatic materialism but even to the more modest and critical form of materialism or naturalism defended by Hayek.